This should do what you want:
sed 's/two.*/BLAH/'
$ echo " one two three five
> four two five five six
> six one two seven four" | sed 's/two.*/BLAH/'
one BLAH
four BLAH
six one BLAH
The $
is unnecessary because the .*
will finish at the end of the line anyways, and the g
at the end is unnecessary because your first match will be the first two
to the end of the line.